1. Field of the Invention
The present invention relates to a fixing device which fixes a toner image to a recording material under application of heat and pressure, an image forming apparatus equipped therewith, and a recording medium on which is recorded the temperature control program.
2. Description of the Related Art
In an electrophotographic image forming apparatus often employed in a copier, laser printer, facsimile apparatus, or the like, a heat fixing method has thus far been common as a fixing method used in a fixing device. A heating roller fixing method using a heating roller is widely employed in this heat fixing type of fixing device. With the heating roller fixing method, the heating roller, inside which is provided a heater which is a heat source, and whose periphery is covered with rubber or resin having a good demoldability, and a pressure roller, are brought into pressure-contact with each other, transfer paper on which is formed a toner image is caused to pass through a nip region formed between these rollers to heat and fuse toner, and the toner is fused onto a surface of the transfer paper, carrying out a fixing. With the heating roller fixing method, the whole of the heating roller is maintained at a certain temperature, so that it is suitable for an increase in speed of printing.
However, in recent years, a full color image forming apparatus such as a laser printer compatible with a full color printing has often been used, and toners of four colors, magenta, yellow, cyan, and black, have been used in the full color image forming apparatus. In the full color image forming apparatus, as there is a need to mix a plurality of kinds of color toner in a condition close to fusion in order to fix a full color toner image, unlike a case of a single color toner fixing in which toner is simply fixed while being softened and pressurized, it is necessary in the fixing device to place toner in a condition in which it is completely fused.
For this reason, with the heating roller fixing type of fixing device in the full color image forming apparatus, an elastic body which is a rubber layer formed of silicone rubber or the like is provided on a support made of a metal or the like with an excellent thermal conductivity, and a surface of the elastic body is covered with a fluorine resin having a good demoldability, forming a heating roller.
However, in the heating roller fixing type of fixing device using this kind of heating roller too, when the image forming apparatus starts to operate, it is necessary to heat the rubber layer with a low thermal conductivity by means of a heat source such as a heater provided inside the heating roller until a predetermined temperature is reached. For this reason, there is a problem in that period of time (a warming-up period of time) needed from the image forming apparatus being powered on until it is operable is lengthened, causing a waiting period of time. Also, there is also a problem in that the temperature of the heating roller decreases at the time of a continuous operation at a high speed.
Thus, in recent years, in order to solve these problems, a fixing device has been proposed wherein, by bringing a belt type external heating section, equipped with a heating belt which rotates while being heated, into abutment with the surface of a heating roller, the heating roller is heated not only from a heater inside it, but also from the external surface (refer to, for example, Japanese Unexamined Patent Publication JP-A 2007-241143 and Japanese Unexamined Patent Publication JP-A 11-24489 (1999)).
As a method of reducing the warming-up period of time in the external heating fixing type of fixing device, the kind of control shown below is described in JP-A 2007-241143.
Heat sources 74 and 75 of external heating members of the fixing device, to be described hereafter in reference to FIG. 4, are powered on. A rotation of a motor is started after the external heating members are heated as far as a first temperature T1. In the event that a belt type external heating member is used as the external heating members, when an external heating belt is at a low temperature, the belt is impressed, and does not rotate easily. For this reason, the motor is rotated after the temperature of the belt is increased to the temperature T1 at which the belt is sufficiently softened. In order to effectively heat the surface of the heating roller, the heat sources 74 and 75 of the external heating members continue to be powered on after the rotation of the motor too. After the belt is heated as far as T3, which is a belt target temperature, an electric power ON/OFF control is carried out so as to maintain the temperature of the belt at T3. When the heat sources of the external heating members are powered off, electric power is switched and applied to a heat source 54 of the heating roller. By repeating this control, the heating roller continues to be heated to its target temperature.
Also, in JP-A 11-24489, there is proposed a fixing device which can maintain a good fixing performance by adjusting the lighting timings of heating sources provided one inside each of a heating fixing member and an external heating member so that the individual heating sources are not lighted at the same time, and by most efficiently using the heating fixing member and external heating member within a limited range of electric power.
Generally, in a case of carrying out the warming-up control described in JP-A 2007-241143, after the external heating members reach the target temperature T3, the external heating members are ON/OFF controlled by the control method shown in FIG. 7 so that they are maintained at T3 and, in the event that a feed of electric power to the external heating members is cut off, electric power is fed to the heat source inside the heating roller in order to efficiently heat the heating roller, and the heating roller is heated from inside. In this case, in order to reduce the warming-up period of time, it is necessary to make electric power high for a lamp inside the heating roller.
Also, normally, in a standby mode, a control of the temperature of the heating roller is carried out by the kind of method shown in the flowchart of FIG. 7, to be described hereafter, (refer to JP-A 11-24489).
It is determined in step S11 whether or not the surface temperature of the heating roller is equal to or less than a predetermined temperature Ta, which is lower than the target temperature and, if it is Ta or less, in step S12, electric power is fed to a halogen lamp. If it is higher than Ta, the process waits.
It is determined in step S13 whether or not the surface temperature of the heating roller has reached Tb, which is the target temperature and, if Tb is reached, in step S14, the feed of electric power to the halogen lamps is stopped. If Tb is not reached, the process returns to step S12, and the heating by the halogen lamp is continued.
However, in the event of making electric power high for the lamp inside the heating roller, when the surface temperature of the heating roller has reached the target temperature, and the feed of electric power to the heat source is cut off, the temperature of the metal core of the heating roller is considerably high. This is a difference in temperature caused by the thermal conductivity of the elastic layer of the heating roller being low. The difference in temperature appears more prominently as the thickness of the elastic layer increases. For this reason, a phenomenon called an overshoot occurs wherein the heat of the metal core is transmitted to the surface of the elastic layer, delayed, even after the feed of electric power to the heater inside the heating roller is cut off, and the surface temperature of the heating roller increases by about 20° C. eventually. Also, by the temperature of the metal core becoming higher, the elastic body in contact with the metal core is exposed to a high temperature, causing a thermal deterioration.
Also, particularly, in a case of shifting from the warming-up operation to the standby mode, the overshoot of the surface temperature of the heating roller is large, and the temperature of the metal core is also at its highest. After the heating roller is idled for a predetermined period of time from a finishing of the warming-up operation, an operational mode switches to the standby mode, stopping the rotation of the drive motor of the heating roller. As the inside of the heating roller is not sufficiently heated at this time, a decrease in temperature of the heating roller occurs at a point at which the heat from the external heating member ceases to be supplied. Thus, electric power is fed to the heat source of the heating roller, carrying out the heating but, as previously described, when a high-power heat source is disposed in order to reduce the warming-up time, in the event that the heating roller is heated by the heat source, causing the surface temperature of the heating roller to reach the target temperature, the metal core portion reaches a considerably high temperature due to a difference in thermal conductivity. Also, period of time needed for feeding electric power to the heat source is lengthened. For this reason, the elastic body in contact with the metal core is exposed to a high temperature, causing the thermal deterioration.
Furthermore, in recent years, a stress on the heating roller has increased because a control reducing a standby mode period of time is used in order to contribute to an energy conservation, and the number of increases in temperature from a low temperature to an operating temperature in one day increases, causing the overshoot of the heating roller, and exposing the elastic body to a high temperature of the metal core.
As a method of suppressing the overshoot and the thermal deterioration of the elastic body, there is a method of reducing the output of the heater lamp inside the heating roller, reducing the output of the heater lamp inside the heating roller at a waiting period of time by means of a phase control, or the like. However, in the event of reducing the output of the heater lamp inside the heating roller, a negative effect, such as an increase in the warming-up period of time, or a decrease in the amount of heat supplied at a mass printing period of time, occurs. Also, it is also possible to reduce the output of the heater lamp by using the phase control, but not only is a ratio of reducing the output limited, but efficiency also decreases. Also, there is a negative effect such that the life of the heater lamp is reduced.